Concrete using Ordinary Portland Cement (OPC) as the binder is the most widely used construction material. The global use of concrete is second only to water, accounting for 70% of all building and construction materials. Although OPC has many advantages, such as ease of application and availability of the raw materials around the world, the production of OPC releases a large amount of greenhouse gases, such as carbon dioxide (CO2). One ton of OPC clinker production emits at least 0.9-0.95 tons of CO2, 60% of which is emitted from the calcination process of limestone with the rest coming from the combustion of fuel in kiln. In fact, cement production in the U.S. accounts for up to 7% of the nation's total CO2 emission. Globally, OPC manufacturing creates even greater percentages of CO2 emissions with up to 10% of the total CO2 emissions in some nations. To combat global climate change, the carbon footprint of OPC-based concrete should be reduced. To this end, two strategies can be used: 1) reducing the amount of OPC used in concrete since OPC is the major contributor of the carbon footprint of concrete; and 2) reabsorbing CO2 into concrete.
Reducing the amount of OPC used in concrete can be realized through partially replacing OPC with supplementary cementitious materials (SCMs) or totally replacing OPC with alternative non-OPC binders, which have lower carbon footprint than OPC, including magnesia cement, sulfoaluminate cements, blended OPC-based cements, and geopolymers. Commonly used SCMs, such as fly ash, grounded blast-furnace slag, and cement kiln dust, are calcium-rich industrial wastes. They can hydrate and/or react with hydration products of OPC and thereby enhance the long-term properties of concrete. However, these reactive SCMs can also create new problems in concrete with respect to retardation, delayed setting time, and low early-age strength.
Non-reactive SCMs, especially ground limestone (mainly consisting of calcite (CaCO3)) are also used to partially replace OPC. Due to the additional surface area provided by the limestone powders for the nucleation and growth of the hydration products, a slight acceleration of the hydration of OPC has been observed with the addition of CaCO3. In addition, CaCO3 can be reactive. It can have limited reactivity with the aluminate phases of OPC. Thermodynamic simulation and experimental studies show that the CaCO3 can alter the hydration products and stabilize ettringite, leading to an increase in the total volume of the hydrate phase, which can reduce the porosity of hardened concrete. Therefore, limited replacement (less than 10%) of OPC by limestone can have some impact on the short and long term performance of concrete. Since some reactive SCMs, such as fly ash, contain an aluminate phase, they can be used together with limestone powders to form blended SCMs. A successful application of such blended SCMs is ternary cement in which blended SCMs consisting of fly ash and limestone can be used to partially replace OPC. Due to the synergistic effect induced by the limited reaction between the limestone powders and aluminate phase in reactive SCMs, the ternary cement using blended SCMs works better than the binder using individual SCMs. However, the use of limestone powder is limited to low replacement levels. At higher replacement levels (more than 10-15% of OPC), most of the limestone is non-reactive and the strength of concrete is reduced due to the dilution effect of the limestone powder. Thus, replacement of OPC with an SCM typically results in some reduction in the strength or the durability of the manufactured concrete.
A second possible way to reduce the carbon footprint in concrete manufacturing is to reabsorb the emitted CO2. CO2 emitted during manufacturing of OPC can be naturally reabsorbed in concrete products through a natural chemical reaction. However, the natural process is relatively slow and it can take hundreds of years to reabsorb all the CO2 during the production of an equivalent amount of concrete. In addition, carbonation is detrimental to concrete because it can cause corrosion of the steel reinforcement present in many concrete applications. However, carbonating concrete at an early age and high concentration and pressure of CO2 can significantly accelerate the strength development of concrete, as shown in some studies. Here, early age concrete specimens are cured in a closed chamber full of CO2 gas. After diffusing into the concrete specimen, CO2 gas can react with fresh concrete and transform into solid calcium carbonates (CaCO3) stored permanently in concrete. Reabsorbing CO2 in concrete is an example of a general concept of storing CO2 permanently in the form of thermodynamically stable carbonates through chemical reaction between CO2 and reactive metal oxides. In addition to cement and concrete, numerous other minerals and industrial wastes have been evaluated to store CO2.
Although using high concentrations and pressures of CO2 can increase the speed of the carbonation, the reaction rate of carbonation can be the major obstacle of this technique. This is because the carbonation reaction rate of early age concrete can be limited by the diffusion of the gaseous CO2 into the concrete matrix, which can be very slow. In addition, the carbonation products, CaCO3 particles, can fill the pores in concrete matrix so that the diffusion of CO2 becomes more difficult as the carbonation reaction progresses. Therefore, existing studies on carbonation curing of concrete are limited to concrete specimens with a small thickness so that diffusion of CO2 to the full depth of the specimen is possible in short period. In addition, the degree of carbonation varies at different depths from the surface and thereby affects the properties of concrete. Excessive carbon curing can destroy calcium silicate hydrate (CSH), the major hydration product and binding agent of OPC, and thereby reduce the strength of concrete. Consequently, the theoretical CO2 absorption of concrete can never be reached if the strength of concrete must be maintained. Also, since a closed curing chamber is needed, carbonation curing technology is usually applicable to only precast concrete.
What are thus needed are new methods and compositions for reducing the carbon footprint associated with concrete manufacture. Such compositions and methods should also permit the strength and durability of the concrete to be maintained, while eliminating the difficulties encountered in existing approaches. The compositions and methods disclosed herein address these and other needs.